Looking at the print and knowing how things go together in production, I suspect that when it's set up for drilling, the operation sheet call out to c'sink while the two pieces are cleco'd together, and it's using the small amount of c'sink that drops into the -5 as additional locating for the part during riveting. In this instance, this is one part of the assembly that the riveting is simply to hold the two parts in correct location for spot riveting and are not structural.
BTW...there are three versions of that drawing, depending on the effectivity of model being built. Structurally they are the same, only small fabrication details are different from what I noticed.